You need to do something to fix your habit of postponing the things you have to do.
Believe me, you'll have a hard time with it once you become a full-fledged member of the society.
ご質問ありがとうございます。
"You need to do something to fix your habit"＝「君は癖を直すために何かする必要がある」
"of postponing the things you have to do."＝「しなければいけないものを後回しにする（癖を）。」
"Believe me,"＝「信じて」
"you'll have a hard time with it"＝「苦労するから」
"once you become a full-fledged member of the society."＝「社会の本格的な一員となったときに。」
ご参考までに。
アメリカ合衆国 
